Experiments on 24-h cycles of sensory performance accuracy and operant behavior output are proposed, emphasizing analyses of free-running circadian and ultradian rhythms under constant environmental conditions, and entrainment patterns in the presence of daily periodic cues. The data will extend previous analyses of general activity rhythms, and physiological functions, to psychophysical detection behavior under the control of weakly discriminable stimuli. Information about the circadian control system, derived from empirical phase-response curves, will be related to periodicities in psychophysical visual stimulus detectability. Concurrent measures will include operant output, detection behavior, food and water ingestion, general motor activity and brain temperature. The entrainment effect of visual stimuli will be compared to that of cycles of food availability and brain-stimulation reinforcer magnitude. Interactions among the oscillating behavioral and physiological subclasses will be documented before and after lesions to the suprachiasmatic nucleus of the hypothalamus, and interpreted in the context of hierarchical models for circadian oscillation systems. BIBLIOGRAPHIC REFERENCES: Terman, M. and Terman, J.S. Circadian functions of reinforcing brain stimulation. In A. Wauquier and E.T. Rolls (Eds.), Brain-Stimulation Reward. Amsterdam: North-Holland, 1976. Terman, M. Circadian oscillation in visual detection behavior in the rat. Paper presented at XXIst International Congress of Psychology, Paris, 1976.